A tablet-type information terminal and a multifunctional mobile phone (so-called smartphone) including a touch screen have become popular. The “touch screen” as used herein refers to a device obtained by combining a touch panel to an image display device such as a liquid crystal display device. Further, the “touch panel” as used herein refers to a planar contact-detection type pointing device. A user directly touches the touch screen with a finger, a stylus, or the like while looking at an image displayed on the screen, thereby performing input.
The generally popularized touch screen is obtained by integrally bonding the image display device and the contact-detection type input device, which are separately formed. Such a touch panel has disadvantages in panel thickness and weight, the number of components, and its cost. Further, the number of manufacturing steps increases, and hence there arises a problem of decreasing yields.
In view of this, there has been proposed a touch screen in which the image display device and the contact-detection type input device are not separately formed but integrally formed. Such a touch screen is referred to as, for example, “embedded type” or “in-cell type”. In this specification, such a touch screen is hereinafter referred to as “embedded touch screen”. Of those, in particular, there has been proposed a system focusing on the fact that the user's finger forms an electrical capacitor. The system detects contact by detecting the change of a capacitance when the finger (or a conductive stylus) touches the surface of the touch screen.
Japanese Patent Application Laid-open No. 2011-227923 discloses an embedded touch screen in which a capacitive element is formed between a common electrode formed on a rear surface of an opposing substrate or a front surface of a pixel substrate and a touch detection electrode formed on a front surface of the opposing substrate. The contact is detected by applying an alternating rectangular wave to the common electrode, and detecting a potential waveform appearing on the touch detection electrode.
In the embedded touch screen disclosed in Japanese Patent Application Laid-open No. 2011-227923, the common electrode is used as an electrode for forming the capacitive element for contact detection. Therefore, the shape of the common electrode is limited. Further, the alternating rectangular wave as a signal for contact detection is used in common with a common drive signal, and hence an applied waveform is not always suitable for contact detection.
This application has been made in view of the above-mentioned circumstances, and has an object to provide an embedded touch screen that is unlimited by the shape of the common electrode and the signal for contact detection.